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Police detained a security guard from the Airbnb where Stefaniak was staying after inconsistencies in his statements, Espinoza said. The man, who is a Nicaraguan national, is being held in jail because he is a flight risk.

The man was staying at the apartment next to Stefaniak's rental, according to the police agency.

A Facebook page that is being updated by a family friend says relatives have not identified the remains.

"The family has yet to see the body and they were in the process of securing dental records for the examiners!" the post on Finding Carla says. "We don't know why the media is announcing the identity of the body or what Costa Rica officials are trying to do by not allowing the family to view the remains BEFORE announcing anything.

Missing for a week

Earlier, police said they had discovered a female body "in a state of decomposition" near where Stefaniak was last seen. They added later that the body was found about 300 meters away from the apartment complex.

The body was found around noon Monday in a wooded area of the San Jose neighborhood of San Antonio de Escazu, he said.

Stefaniak, 36, was reported missing by her family in Florida last week. Relatives raised the alarm after she stopped texting and telephoning home and was not on her return flight from Costa Rica, where she had been vacationing.

Authorities found stains inside the apartment where Stefaniak was staying, "which are compatible with blood and which will be submitted to further investigation and comparison," Espinoza said.

Birthday celebration

The last time anyone heard from Stefaniak was the night of November 27, according to April Burton, her sister-in-law.

Burton told CNN the two were in Costa Rica to celebrate Stefaniak's birthday, and had traveled together for six days.

Burton left the country November 27, and Stefaniak's flight back to Florida was scheduled for the next day.

The two texted each other via WhatsApp throughout the day that Tuesday, Burton said, until Stefaniak suddenly stopped responding to her messages that evening.

And Wednesday, Stefaniak did not board her flight home.

"That's when we knew that something's not right," Burton said. "You never want to think the worst, but that's where your mind goes to automatically. We're going on six days not hearing from her. That's kind of where we are right now."

Unusual behavior

Burton added: "For almost six days to go by and no contact, she would have to have been abducted. She is very active on social media, and for her, on the day of her birthday, to never log in to any of those accounts is just crazy."

A US State Department representative told CNN that officials are aware of the reports of a missing American in Costa Rica.

"When a US citizen is missing, we work closely with local authorities as they carry out their search efforts. We stand ready to provide all possible assistance to US citizens in need and to their families. Due to privacy considerations, we have no further comment," the representative said.

"She's a great person. So friendly, so bubbly, loves to travel, loves fashion, loves social media," she said.

CNN affiliate WFTS reported that a lawyer for the place where Stefaniak was staying said the facility is temporarily closed.

"The owners are fully cooperating with the authorities in the investigation and we hope that once the facts are determined, the perpetrator is brought to justice," the attorney, who was not identified, told the Tampa station.

CORRECTION: This story has been updated to correct the day the body was found and to correct the timeline of Stefaniak's last communication and scheduled travel day.